In normal lung, the predominant cytoplasmic
carbonic anhydrase (CA)
isozyme (CAII) is highly expressed in
amine- and
peptide-producing pulmonary neuroendocrine cells where its role involves CO2 sensing. Here, we report robust cytoplasmic expression of CAII by immunohistochemistry in the
tumor cells of different native
neuroendocrine tumor (NET) types, including typical and atypical
carcinoids and small-cell lung
carcinomas, and in NET and non-NET tumor cell lines. Because, in both pulmonary neuroendocrine cell and related NETs, the
hypercapnia-induced secretion of bioactive
serotonin (5-hydroxytryptamine) is mediated by CAII, we investigated the role of CAII in the
biological behavior of
carcinoid cell line H727 and the type II cell-derived A549 using both in vitro clonogenicity and in vivo xenograft model. We show that
short hairpin RNA-mediated down-regulation of CAII resulted in significant reduction in clonogenicity of H727 and A549 cells in vitro, and marked suppression of
tumor growth in vivo. CAII-
short hairpin RNA cell-derived xenografts showed significantly reduced mitosis (phosphohistone H3 marker) and proliferation associated
antigen Ki-67 (Ki67 marker), and significantly increased apoptosis by
terminal deoxynucleotidyl transferase dUTP nick end labeling assay. Using an apoptosis gene array, we found no association with
caspases 3 and 8, but with a novel association of CAII-mediated apoptosis with specific mitochondrial apoptosis-associated
proteins. Furthermore, these xenografts showed a significantly reduced vascularization (CD31 marker). Thus, CAII may play a critical role in NET lung
tumor growth, angiogenesis, and survival, possibly via
5-hydroxytryptamine, known to drive autocrine
tumor growth. As such, CAII is a potential therapeutic target for the difficult-to-treat lung NETs.